Dowsing for nodal lines in a topological semimetal

In PNAS, Shao et al. (1) report a spectroscopic search for line degeneracies in the electronic bands of NbAs₂, a topological semimetal (TSM). TSMs have attracted great interest recently as exemplars of the emergent physics that can arise in quantum condensed matter (2). They host quasiparticle excitations that cannot occur in the vacuum but do occur in solid-state realizations when protected by discrete symmetries (3). This search is particularly fascinating because the spectroscopic features they uncover are, by any reasonable measure, completely hidden. First, the nodal lines in question are energy dispersive and nearly completely submerged within a Fermi sea, where they are inaccessible to transport measurements. Second, even if the nodal lines could be energy-flattened and floated to the Fermi surface, they would remain hidden. This is because spin-orbit coupling in this material removes the Dirac line degeneracy on an experimentally relevant energy scale. The title of their paper, “Optical signatures of Dirac nodal lines in NbAs₂,” is thus a curious misnomer, since there are actually no Dirac nodal lines in this material! Nonetheless, by using infrared spectroscopy, this group detects the fingerprints of these hidden features, which turn up in frequency-scaling relations for the infrared conductivity. One can compare their method to dowsing, the folklore practice of searching for underground water, minerals, or anything invisible indirectly by observing the motion of a pointer or the changes in direction of a pendulum (https://en.wikipedia.org/wiki/Dowsing). In the case of Shao et al. (1), their method actually works and is interpreted using a theory of the scaling relations. As a bonus, the authors suggest that the nascent nodal lines might explain an unusually strong magnetoresistance of the family of related dipnictides, XPn₂, where X = {Ta, Nb} and Pn = {P, As, Sb} (4⇓– …

↵¹Email: mele{at}physics.upenn.edu.